The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
One goal of a network manager is to control total cost of ownership of the network. Cabling problems can cause a significant amount of network downtime and can require troubleshooting resources, which may increase the total cost of ownership. Providing tools that efficiently solve cabling problems may increase network uptime and reduce the total cost of ownership.
In wireline networks, such as Ethernet-based networks, network devices may be interconnected using multi-conductor cables. A network device generally comprises a physical layer module (PHY) and a medium access controller (MAC). The PHY may connect the network device to a cable. The MAC provides an interface between the PHY and a host.
Referring now to FIG. 1, a MAC 2 and a PHY 4 in a network device 10 communicate via a two-wire management data input/output (MDIO) interface. The MDIO interface is a synchronous serial interface that the MAC 2 uses to access internal registers Reg. 1, Reg. 2, . . . , Reg. N of the PHY 4. Specifically, the MAC 2 reads and writes data into the internal registers of the PHY 4 via the MDIO interface. The MDIO interface is defined by I.E.E.E. section 802.3 and is incorporated herein by reference in its entirety.
The MDIO interface comprises a management data clock and a MDIO bi-directional multi-drop data line. A MDIO control module 6 provides the management data clock to synchronize data transfers on the data line. The data line is a shared tri-stateable bus. A MDIO interface module 8 in the PHY 4 interfaces the internal registers of the PHY 4 to the MDIO control module 6. The MAC 2 drives the data line during write operations, and the PHY 4 drives the data line during read operations. A processor 14 in the MAC 2 controls the read/write operations, processes data, and communicates with a host (not shown).
Referring now to FIGS. 2-3, a first network device 20 may be connected to a second network device 22, called a link partner, using a cable 16. The first network device 20 comprises a PHY 24 and a MAC 26. The PHY 24 may be coupled to the cable 16 via a connector 12. The second network device 22 comprises a PHY 28 and a MAC 29. The PHY 28 may be coupled to the cable 16 via a connector 15. The cable 16 may be a CAT 5 or a CAT 6 twisted-pair cable having four pairs of twisted wire as shown in FIG. 3 (twist not shown) or other suitable cable. Connectors 12 and 15 may be RJ45 connectors or other suitable connectors.
Referring now to FIG. 4, a conventional cable tester 30 may be frequently used to isolate cabling problems. The cable tester 30 is coupled by the connector 12 (such as a RJ-45 or other connector) to the cable 16. The connector 15 connects the cable to a load 36. The load 36 is typically a loopback module. The cable tester 30 detects a short or an open condition in a crossed or a reversed pair in the cable 16. The short or open condition may be detected without the load 36. Additionally, the cable tester 30 can determine a length of the cable 16 and a distance from one end of the cable 16 to a point where the cable 16 may have a fault such as a short or an open condition.
In a multi-conductor cable, a short condition may occur when two or more conductors in the cable 16 are short-circuited together. An open condition may occur when one or more conductors in the cable 16 lack continuity between both ends of the cable 16. A crossed pair may occur when a pair of conductors communicates with different pins at each end of the cable 16. For example, a first pair may communicate with pins 1 and 2 at one end and with pins 3 and 6 at the other end. A reversed pair may occur when two ends in a pair are connected to opposite pins at each end of the cable 16. For example, a conductor connected to pin 1 on one end may communicate with pin 2 at the other end, and a conductor connected to pin 2 on one end may communicate with pin 1 at the other end.
The cable tester 30 may employ time domain reflectometry (TDR), which is based on transmission line theory, to troubleshoot faults in the cable 16. The cable tester 30 may transmit a pulse 37 on the cable 16 and analyze a reflection or a return pulse 38 when received. Specifically, the cable tester 30 may measure a difference between a time when the pulse 37 is transmitted and a time when the return pulse 38 is received. Additionally, the cable tester 30 may analyze characteristics such as shape and size of the return pulse 38 relative to the pulse 37 that is transmitted. Based on the characteristics of the pulses and electrical properties of the cable 16 such as specific resistance, propagation constant, etc., the cable tester 30 identifies faults in the cable 16. Additionally, the cable tester 30 estimates the length of the cable 16 and a distance from a test point to a point of a fault in the cable 16.
Conventional cable testers 30, however, may generate inaccurate results when the cable 16 is terminated by an active link partner generating link signals during a test. For example, TDR cannot determine the length of the cable 16 (hereinafter cable length) when the link is active, that is, when the link partner at the remote end of the cable 16 is active or in use. This is because the remote end of the cable 16 is properly terminated when the link partner is active. In that case, the cable 16 functions as a substantially balanced transmission line. That is, when the remote end receives a TDR pulse, the remote end may return a very weak signal. Weak return signals cannot be analyzed unless extensive electronic circuits are used. Implementing extensive electronic circuits, however, can be expensive and may not be feasible in low-cost systems.
On the other hand, digital signal processing (DSP) can determine cable length when the link is active. In DSP, unlike in TDR, no pulses are injected into the cable 16. Instead, parameters such as amplitude, pulse width, pulse shape, etc., of signals that are normally transmitted and received on the cable 16 are measured to determine the length of the cable 16. DSP, however, involves making some assumptions and therefore yields cable length measurements that are approximate rather than accurate.
For example, if the cable length is determined based on amplitude of a received signal, the amplitude of the transmitted signal is generally unknown or unknowable and therefore needs to be assumed. Additionally, any attenuation in the received signal is calculated by assuming an average attenuation per unit length of the cable 16. Therefore, cable length determined using DSP is generally an approximate estimate rather than an accurate measurement.
Thus, in low-cost cable testers, since TDR cannot analyze reflections from a terminated or an active remote end, TDR cannot determine cable length although the cable 16 is good, i.e., although the cable 16 has no fault. On the other hand, although DSP can determine the length of the cable 16 that is properly terminated or connected to an active remote end, DSP fails to determine the length if the cable 16 is too long for local and remote ends to communicate.